For a condenser microphone unit, incorporated parts are fixed to a unit case by staking (curling) or the like means. The microphone unit of this type incorporates an impedance converter. For example, Japanese Patent Application Publication No. 2006-287326 has disclosed a condenser microphone unit of this type. A conventional example of the condenser microphone unit is explained with reference to FIGS. 3 and 4. FIG. 3 is a sectional view showing a state before the unit case is staked, and FIG. 4 is a sectional view showing a state after the unit case has been staked.
As shown in FIG. 3, this condenser microphone unit 1 has a bottomed cylindrical unit case 2, and an acousto-electric converter 3 is accommodated in the unit case 2.
The unit case 2 is formed by a press-molded product of a metallic material such as aluminum, and is provided with a front acoustic terminal 21 on the front end surface side thereof (the surface side directed to the sound source side when sound is picked up) corresponding to a bottom part. The rear end surface side of the unit case 2 forms an opening part 22 through which the acousto-electric converter 3 is accommodated. This condenser microphone unit 1 is ominidirectional because of having no rear acoustic terminal.
The acousto-electric converter 3 includes a diaphragm 31 stretchedly provided on a support ring (diaphragm ring) 32 and a backplate 33 fixed on the front surface side (in FIG. 3, on the lower surface side) of a cylindrical insulating seat 34 formed of a synthetic resin, and the diaphragm 31 and the backplate 33 are disposed so as to face each other via a spacer ring 35.
In the opening part 22 on the rear end surface side of the unit case 2, a circuit board 5 is disposed so as to close the opening part 22. Although not shown in the figures, the circuit board 5 is formed with solder lands to which a drain electrode 42 and a source electrode 43, described later, are soldered and a ground (earth) pattern, to which a feeder line, a signal line, and a shield braided wire (all not shown) of a microphone cable, not shown, are connected.
This condenser microphone unit 1 includes a FET 4 serving as an impedance converter. In this example, a recess 341 is formed in the insulating seat 34, and the FET 4 is accommodated in the recess 341. The FET 4 is sometimes mounted on the inner surface side of the circuit board 5.
A gate electrode 41 of the FET 4 is electrically connected to the backplate 33, and the drain electrode 42 and the source electrode 43 are soldered to the predetermined solder lands of the circuit board 5. In FIGS. 3 and 4, only one of the drain electrode 42 and the source electrode 43 is shown in the figures because these elements are located at lapping positions on the drawing.
The condenser microphone unit 1 is assembled as described below. First, the acousto-electric converter 3 is housed in the unit case 2, thereafter a rubber ring 6 being displaced on the rear surface side (in FIG. 3, on the upper surface side) of the insulating seat 34, and the circuit board 5 is disposed on the rubber ring 6 while the drain electrode 42 and the source electrode 43 are inserted therethrough.
Then, after an edge 221 of the opening part 22 on the rear surface side is staked while being curled toward the inside as shown in FIG. 4, the drain electrode 42 and the source electrode 43 are soldered to the circuit board 5. Thereby, the rubber ring 6 is compressed to a proper degree, and the incorporated parts including the acousto-electric converter 3 and the circuit board 5 are fixed in the unit case 2.
As described above, the condenser microphone unit 1 incorporates the FET 4. Therefore, for example, when a cellular phone is used near the microphone, high-frequency signals produced by considerably strong electromagnetic waves radiated from the cellular phone are detected by the FET 4, so that noise of audible frequency may be generated.
To prevent the noise generation, in this conventional example, the FET 4 is sealed by a sealant material 7 for preventing EMI.
However, if large amounts of strong electromagnetic waves radiated from the cellular phone or the like are applied to the microphone cable connected to the circuit board 5, a high-frequency current caused by the applied electromagnetic waves may intrude into the unit case 2 through the microphone cable, and may be detected by the FET 4. Therefore, the problem that noise of audible frequency is generated is not yet solved.
Besides the above-described problem, there arises another problem that some of sound waves entering through the front acoustic terminal 21 may leak. The reason for this is that though the opening part 22 on the rear surface side of the unit case 2 is sealed by the compression of the rubber ring 6, a space is present on the inside of the rubber ring 6, or that the airtightness between the outer peripheral surface of the rubber ring 6 and the inner peripheral surface of the unit case 2 is insufficient in some cases.
Also, there arises a still another problem that since the rubber ring 6 is partially in contact with the incorporated parts, a steady internal stress cannot be applied especially to the acousto-electric converter 3 uniformly in some cases.
Accordingly, an object of the present invention is to provide a condenser microphone unit incorporating a FET (impedance converter) in a unit case, in which the electromagnetic shield on the rear surface side of the unit case is assured, sound waves do not leak, and a steady internal stress is applied to incorporated parts almost uniformly.